Some legal scholars dispute that claim, while others think it’s dead on. About the only point everyone agrees on is that the courts have never decided this exact issue. We’ll walk you through some of the leading arguments on both sides.

The Antiquities Act

Passed in 1906, the Antiquities Act gave presidents the power to protect any tract of property owned or controlled by the federal government in order to preserve "historic landmarks, historic and prehistoric structures, and other objects of historic or scientific interest." At the time, the main goal was to keep Native American archeological sites safe for future study. Within a few decades though, the scope grew to include broader land conservation purposes.

The law is very short, and the only limit it places on a president is the amount of land "shall be confined to the smallest area compatible with proper care and management of the objects to be protected."

Udall’s office cited an analysis by the Congressional Research Service, the nonpartisan policy arm of Congress. That report said under the 1906 law, a president might be "constrained" in his or her ability to reverse what an earlier president has done.

"No President has ever abolished or revoked a national monument proclamation, so the existence or scope of any such authority has not been tested in courts," the report said. "However, some legal analyses since at least the 1930s have concluded that the Antiquities Act, by its terms, does not authorize the President to repeal proclamations, and that the President also lacks implied authority to do so."

We reached several legal experts. Three of them -- Mark Squillace at the University of Colorado, John Ruple at the University of Utah and Michael Blumm at Lewis and Clark Law School -- said that summary was correct.

Two other experts, John Yoo at the University of California Berkeley and Todd Gaziano at the Pacific Legal Foundation, a self-described "conservative/libertarian public interest law firm," disagreed.

Why Trump might be thwarted

Squillace told us that the Antiquities Act is silent on a president’s power to withdraw a designation. If Congress had wanted to allow that, based on other bills passed at the time, Squillace said it would have done so.

He said two laws, the Pickett Act of 1910 and a 1897 Forest Service law, specifically said a president "may vacate altogether" any order that set aside land under those acts.

Ruple echoed that point and said Congress "clearly knew that was an option and how to do it, if they had wanted to."

Experts who think the law stands in Trump’s way also cite a 1938 opinion from Attorney General Homer Cummings. Squillace said Cummings told President Franklin Delano Roosevelt that "the Antiquities Act ‘does not authorize (the President) to abolish (national monuments) after they have been established.’"

Trump would need to deal with a more recent law, these law professors said. The 1976 Federal Land Policy and Management Act says the Interior Secretary "shall not .. modify or revoke any withdrawal (to protect land) creating national monuments under the Act of June 8, 1906."

Basically, the argument goes that through both the 1906 and the 1976 laws, Congress was making sure that it retained final say over protected lands. If a president protected a block of land, only Congress had the authority to reverse it.

Why Trump might prevail

Yoo and Gaziano argued in a paper for the American Enterprise Institute that the Constitution gives Trump the right to rescind a national monument designation.

"Under Article I of the Constitution, only Congress can enact domestic statutes with any degree of per­manence," they wrote. "A basic principle of the Constitution is that a branch of government can reverse its earlier actions using the same process originally used."

In response to the argument that the Antiquities Act says nothing about revoking a designation, they noted that the Constitution is similarly silent about passing laws in general. It grants Congress the power to make laws, but there’s no explicit power for it to undo them. (Except by passing a new law.)

They also said that no president can bind a future president.

"Presidents commonly issue executive orders reversing, modifying, or even extending the executive orders of past presidents, and no court has ever questioned that authority, even when it is used to implement statutorily delegated powers," they wrote.

As for that 1976 land management law, Gaziano said it only applies to the interior secretary, not the president.

The option to change the boundaries

Udall spoke only about the power to rescind a national monument designation, not reduce the size of the territory protected. There are many cases when presidents have changed the boundaries.

Yoo and Gaziano gave several examples. President Dwight Eisenhower reduced the reservation for the Great Sand Dunes National Monument by 25 percent. President Harry Truman took away about half of the 9,500 acres protected under the Santa Rosa Island National Monument.

There’s no dispute that this is possible. The legal question is, what are the limits?

Bloom at Lewis and Clark Law School said this is a gray area.

"When you create the monument you give reasons and purposes for creating it," Bloom said. "In any subsequent change, those purposes have to be explained as still being fulfilled. But we don’t have any legal history on what that means.

Bloom said, if Trump does anything to alter a national monument, a court challenge is guaranteed, "and we won’t get an answer for a couple of years."

One last point: Congress can do whatever it wants by passing a law. The debate here is simply over the power of the president. Trump and others have an expansive view and eventually, we’ll find out whether they carry the day.

Email interview, Mark Squillace, Professor of Law, University of Colorado Law School, April 27, 2017

Email interview, John Ruple, research associate professor, College Of Law, University of Utah, April 27, 2017

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